This application claims the benefit of Japanese Patent Application Nos. 10-140404 and 10-140405, both filed on May 8, 1998, which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a reflective mirror that is used where large quantities of heat are absorbed from incident light rays or X-rays and more particularly to an optical system of a projection exposure apparatus using light or soft X-rays such as a soft X-ray projection exposure apparatus used in the manufacture of semiconductor devices, etc.
2. Description of the Related Art
In recent years, as semiconductor integrated circuit elements have become smaller in size, projection-lithographic techniques using X-rays with shorter wavelengths instead of conventional ultraviolet light have been developed in order to improve the resolution of optical systems limited by the boundaries of light diffraction. X-ray projection exposure apparatuses used in such techniques are constructed mainly from an X-ray source, an illumination optical system, a mask, a focusing optical system and a wafer stage, etc.
Syncrotron-radiation (SR) light sources or laser plasma X-ray sources are used as X-ray sources. Illumination optical systems are constructed from grazing-incidence mirrors, multi-layer film mirrors and filters that reflect or transmit only X-rays of a specified wavelength, etc., and are used to illuminate the mask with X-rays of a specified wavelength.
Masks include transmission type masks and reflection type masks. Transmission type masks are masks in which a pattern is formed by disposing a substance that absorbs X-rays in a specified shape on the surface of a thin membrane consisting of a substance that shows good transmission of X-rays.
Reflection type masks are masks in which a pattern is formed (for example) by disposing parts that have a low reflectivity in a prescribed shape on the surface of a multi-layer film that reflects X-rays. The patterns formed on such masks are focused on the surface of a wafer coated with a photo-resist by means of a projection and focusing optical system constructed from a plurality of multi-layer film mirrors, so that these patterns are transferred to the above-mentioned photo-resist. Furthermore, since X-rays are absorbed by the atmosphere and thus attenuated, the entire light path is maintained at a specified degree of vacuum.
In the X-ray wavelength region, transparent substances do not exist. Furthermore, the reflectivity at the surfaces of substances also is extremely low. Accordingly, ordinary optical elements such as lenses and mirrors, etc., cannot be used. Optical systems used for X-rays are constructed by means of grazing-incidence mirrors which reflect X-rays by utilizing the total reflection of X-rays incident on the reflective surface from an oblique direction, and multi-layer film mirrors in which the phases of the reflected X-rays coincide at the respective interfaces of a multi-layer film. Thus, a high reflectivity is obtained by means of an interference effect, etc.
Since grazing-incidence optical systems have a large aberration, a resolution at the diffraction limit cannot be obtained. On the other hand, multi-layer film mirrors are capable of reflecting X-rays perpendicularly, so that an X-ray optical system at the diffraction limit can be constructed. Accordingly, the focusing optical systems of soft X-ray projection exposure apparatuses are all constructed from multi-layer film mirrors.
In such X-ray multi-layer film mirrors where a multi-layer film consisting of molybdenum and silicon is used on the long-wavelength side at the L absorption end of silicon (12.3 nm), absorption by silicon is decreased so that the maximum reflectivity can be obtained. Nevertheless, the reflectivity at wavelengths of 13 to 15 nm is about 70% regardless of the angle of incidence. Recently developed multilayer film consisting of molybdenum and beryllium exhibits the maximum reflectivity on the long-wavelength side at the K absorption end of beryllium (11.0 nm). The reflectivity at wavelengths of 11 to 12 nm is about 70% regardless of the angle of incidence. At wavelengths shorter than the K absorption end of beryllium, hardly any multi-layer films that allow a reflectivity exceeding 30% to be obtained at perpendicular incidence have been developed.
Glass materials such as quartz, etc., which have a high shape precision, a small surface roughness, and can be worked, are used as substrate materials in multi-layer film mirrors.
In order to obtain a practical through-put (e. g., about 30 wafers per hour in the case of 8-inch wafers) in X-ray projection exposure apparatuses of the type described above, it is necessary to irradiate the surfaces of the multi-layer film mirrors making up the focusing optical system with X-rays of a certain intensity (e. g., about 10 [mW/cm2]). On the other hand, as was mentioned above, the reflectivity of such multi-layer film mirrors is about 70% at the most. The remaining X-rays are absorbed, transmitted or scattered without being reflected by the multi-layer film. The loss due to scattering is slight, and the X-rays passing through the multi-layer film are more or less completely absorbed by the substrate.
Specifically, most of the X-rays that are not reflected by a multi-layer film mirror are absorbed by the multi-layer film mirror, so that the energy of these X-rays is converted into heat. The temperature of the multi-layer film mirror is elevated by this heat, so that thermal deformation occurs.
Generally, in order to obtain a resolution at the diffraction limit in an optical system, the shape error of the mirrors and lenses making up the optical system must be sufficiently small compared to the wavelength of the light used. Furthermore, in an optical system using X-rays, the permissible range of shape error is narrower than that in optical systems using visible light or ultraviolet light by an amount corresponding to the shortening of the wavelength. Viewed in this way, the thermal deformation of the multi-layer film mirrors caused by the above-mentioned X-ray irradiation has a major effect on the focusing characteristics of the multi-layer film mirrors. Accordingly, there is a danger that the designed resolution cannot be obtained.
Consequently, the cooling of such mirrors from the underside of the substrate of each mirror is performed in order to prevent any thermal deformation effect on the focusing characteristics. However, a sufficient effect cannot be obtained in such methods. Furthermore, since X-ray optical systems are used in a vacuum, there is almost no heat conduction to environment from the surfaces of the mirrors.
Accordingly, in order to prevent the effects of thermal deformation on the focusing characteristics, it is necessary to limit the intensity of the X-rays incident on the mirrors. If this is done, however, the through-put of the soft X-ray projection exposure apparatus using these mirrors drops. In other words, a problem of conventional mirrors is that high resolution and high through-put of the soft X-ray projection exposure apparatus could not be simultaneously achieved.
Problems encountered in the X-ray optical system of a soft X-ray exposure apparatus have been described above. However, problems caused by thermal deformation of reflective mirrors are also encountered to a varying degree in other X-ray optical systems and in optical systems using light rays in wavelength regions other than the X-ray wavelength region.
The present invention is directed to a reflective mirror that has a small shape error and surface roughness, and can sufficiently suppress thermal deformation caused by irradiating electromagnetic radiation such as X-rays, light, etc.
An object of the present invention is to provide a method for manufacturing a reflective mirror with reduced thermal deformation.
Still a further object of the present invention is to provide a soft X-ray projection exposure apparatus that makes it possible to achieve both a high resolution and a high through-put.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention include a soft X-ray projection exposure apparatus having at least one metal mirror. The mirror constitutes at least one of an illumination optical system and a projection optical system. The mirror includes a metal substrate having a front surface and a rear surface. A thin film of an amorphous substance is formed on the front surface of the metal substrate. A front surface of the amorphous substance is polished to optical smoothness. A multi-layer film is formed on the front surface of the thin film. The multi-layer film reflects X-rays of a specified wavelength.
In another aspect of the invention, a method for manufacturing a mirror is provided. A metal substrate is prepared. An amorphous thin film containing a nickel alloy as a chief ingredient is formed on a surface of the metal substrate. A surface of the amorphous thin film is worked into an optically smooth surface.
In another aspect, the invention includes a mirror for use when large amounts of heat from incident electromagnetic radiation is absorbed. The mirror includes a metal substrate having a front surface and a back surface. A thin film of an amorphous substance is formed on the front surface of the substrate. The thin film has a surface polished to optical smoothness.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only are not restrictive of the invention, as claimed.